Modulated electromagnetic stirring of metals at advanced stage of solidification

ABSTRACT

A method and apparatus for electromagnetic stirring of molten metals at an advanced stage of solidification, as may be used in continuous casting of steel billets and blooms, are disclosed. At least first and second stirrers are provided for generating first and second rotating magnetic fields of a differing frequency about an axis of solidifying molten metal. The stirrers are arranged about the molten metal in sufficiently close proximity to each other so that their respective magnetic fields superpose to produce a modulated magnetic field. The magnetic fields of the respective stirrers may either have common or opposing rotational directions. The modulated stirring produced by the magnetic fields results in oscillating primary and secondary flows and hence turbulence within the melt bulk in the region wherein temperature of the melt on its central axis is below the liquidus level and at least 10% of substantially solidified material is formed. Turbulent flow created by this stirring method disrupts formation of crystalline structures in the melt bulk and mixes solute enriched melt of the central region with the bulk volume which subsequently results in improvements of the solidification structure and overall internal quality of the cast products.

FIELD OF THE INVENTION

The present invention relates to electromagnetic stirring and more particularly to electromagnetic stirring of liquid metals as they solidify. The invention may be used in continuous casting of steel, alloys, or other metallic melts, and in other solidification processes of these materials.

BACKGROUND OF THE INVENTION

Electromagnetic stirring (EMS) is commonly used in the production of continuously cast steel billets, blooms, and the like; the casting of different alloys; and other liquid metal casting and processing. Typically, A.C. electric current is applied to induction coils that surround the melt. The A.C. current excites a continuous rotating A.C. electromagnetic field that stirs a metal, such as in the production in continuous cast steel billets and blooms. For example, the A.C. field may stir the melt in the continuous casting mold, at an early stage of solidification.

Rotational stirring of the melt within the mold produces turbulence and shear force at the solid-liquid interface. This results in fragmentation of dendrites at the solidification front and the formation of an equiaxed solidification structure, which is the most important objective of stirring in the mold.

EMS may also be used for stirring the unsolidified portion of the continuously cast strand below the casting mold, at a later, or an advanced solidification stage.

Conventional rotational stirring, however, is not effective at an advanced stage of solidification of the melt, because any turbulence produced by rotational stirring is essentially limited to the solid-liquid interface.

In an effort to improve the effectiveness of rotational stirring, Japanese Patent Publications No. 52-4495 and No. 53-6932, and Kojima et al., Application of Advanced Mild Stirring to a Bloom Caster (The Latest Kosmostir-Magnetogyr Process Technique) describe intermittent and alternating rotational stirring. Intermittent stirring is achieved by applying an electric current intermittently to energize stirring coils. Alternating stirring is produced by generating a magnetic field that alternates its rotating direction. The effectiveness of intermittent and alternating stirring, however, has proven to be limited since they do not produce significant turbulence in the melt beyond the solid-liquid interface. In addition, the total stirring time available for stirring of continuously cast billets and blooms is limited by the 10 to 40 second period, depending on the cast product cross-section size and the related casting speed. This relatively short time period will restrict both the duration and the number of intermittent or alternating stirring cycles. The alternating stirring can also be performed without dormant periods.

Other methods of EMS rely on magnetic field modulation resulting from the application of electric current of varying frequency and/or amplitude using a programmable power source. Such an EMS method is, for example, described in U.S. Pat. No. 4,852,632. As disclosed, this method may create a “gentle” stirring by gradually changing the stirring flow direction in order to avoid or reduce the formation of negative segregation at the stirring pool boundary in continuously cast blooms. Similar methods of magnetic field modulation have been described in (Ref. H. Branover et al., the U.S. Patent Application No. US2007/0157996A1, J. Pal et al., the German Patent DE 102004017443). These modulation methods have proved to be effective at modulation periods of about 10 seconds, which also limits their usefulness in continuous casting of billets and blooms.

Accordingly, there is need for new EMS methods and apparatus generating greater turbulence.

SUMMARY OF THE INVENTION

In accordance with the present invention, an EMS method and apparatus generating greater turbulence in the solidifying melt volume is provided. Specifically, an applied magnetic field is formed by juxtaposing and thereby modulating at least two independent fields of different frequency to produce turbulent EMS. The method and apparatus are particularly suited for stirring at advanced stages of solidification.

In accordance with an aspect of the present invention, there is provided a method of electromagnetic stirring a molten metallic material. The method comprises: providing at least two stirrers for generating independent rotating magnetic fields about an axis extending through the molten material. At least first and second ones of the at least two stirrers produce independent first and second rotating magnetic fields have differing angular frequencies. The stirrers are located about the molten metallic material in sufficiently close proximity to each other so that the independent rotating magnetic fields superpose to produce a modulated magnetic field that creates a turbulent flow of the molten metallic material in a transition region of the molten metallic material having a temperature below the liquidus along a central axis of the molten metallic material, and in which the molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material.

In accordance with another aspect of the present invention, there is provided a casting apparatus. The casting apparatus comprises a mold for casting a molten metal; a first stirrer for generating a first rotating magnetic field about an axis extending through the molten metal, located downstream of the mold; a second stirrer for generating a second rotating magnetic field, located downstream of the first stirrer; at least one power source for generating the first and second magnetic field, at frequencies of rotation differing from each other; wherein the first and second stirrers are arranged in proximity to each other so that the first and second rotating magnetic fields produce a modulated magnetic field that creates a turbulent flow in a molten metallic material in a region between the first and second stirrers.

In accordance with another aspect of the present invention, there is provided a method of electromagnetic stirring a metallic melt. The method comprises: providing a first stirrer for generating a first rotating magnetic field that rotates about an axis extending through the melt, at an angular frequency of ω₁; providing a second stirrer for generating a second rotating magnetic field that rotates at an angular frequency of ω₂. The first and second stirrers are located in sufficiently close proximity to each other so that the first and second rotating magnetic field produce a magnetic force having a frequency component with frequency (ω₁−ω₂) in the metallic melt in a region between the first and second stirrer, wherein (ω₁−ω₂) is sufficiently small to allow the magnetic force to overcome the inertia of the melt.

In accordance with yet another aspect of the present invention there is provided a method of electromagnetic stirring a molten metallic material. The method comprises: providing a first stirrer for generating a first rotating magnetic field about an axis extending through the molten material; providing a second stirrer for generating a second rotating magnetic field having a frequency of rotation differing from the first rotating magnetic field; wherein the first and second stirrers are located about the molten metallic material in sufficiently close proximity to each other so that the first and second rotating magnetic fields superpose between the first and second stirrers to produce a modulated magnetic field that creates a turbulent flow of the molten metallic material in a transition region of the molten metallic material having a temperature below the liquidus along the axis, and in which the molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate by way of example only, embodiments of the present invention,

FIG. 1 is a schematic cross-sectional view of an EMS apparatus on a continuous casting machine, exemplary of an embodiment of the present invention;

FIG. 2 is a schematic cross-section view of an example stirrers of the EMS apparatus of FIG. 1;

FIG. 3 is a simplified perspective view of a stirrer of FIG. 1;

FIG. 4 is a schematic solidification profile, illustrating isolines of solid fraction, of a portion of a cast strand formed by the casting machine of FIG. 1, in a liquid-to-solid transitional region (the “mushy zone”);

FIG. 5 is a graph of example axial profiles of magnetic flux density produced by two adjacent stirrers of the EMS apparatus of FIG. 1;

FIG. 6 is a graph of modulated magnetic force resulting from the superposition of two example magnetic fields of the same rotating direction;

FIG. 7 is a graph of a low frequency component of magnetic force resulting from filtering the force of FIG. 6 by the melt inertia;

FIG. 8 is a graph of angular velocity produced by modulated stirring resulted from superposition of two magnetic fields of the same rotating direction in an example (e.g. mercury) melt.

FIG. 9. is a graph of axial profiles of angular stirring velocities produced by different modes of stirring, in the example melt;

FIG. 10 is a graph of example angular velocities produced by modulated counter-rotating stirring in the example melt;

FIG. 11 is graph of stirring velocity profiles at the locales along a central axis of an exemplary melt of steel;

FIG. 12 is a schematic representation of the melt locales at which axial stirring velocity and turbulent viscosity of FIG. 11 were determined by a 3-dimensional numerical simulation;

FIG. 13 is a graph of example of turbulent viscosity at different locales of the stirring pool central axis produced by modulated counter-rotating stirring; and

FIG. 14 is a graph of example of turbulent viscosity at different locales of the stirring pool central axis produced by conventional, unidirectional stirring.

DETAILED DESCRIPTION

FIG. 1 is a schematic cross-sectional view of a continuous casting machine 10, including an EMS system 12, exemplary of an embodiment of the present invention. Casting machine 10 includes a tundish 14, from which a molten metal, such as liquid steel, or the like is transferred into a casting mold 18 through a submerged entry nozzle 20. Within mold 18, a cast strand 22 having an outer shell surrounding a melt 41 takes form. Cast strand 22 exits from the bottom of mold 18.

Example EMS system 12, typically includes at least one electromagnetic stirrer 24 arranged about mold 18. Stirrer 24 could be arranged within the mold housing, or may be enclosed in a housing (not shown) surrounding the mold. As will become apparent, stirrer 24 is arranged to induce stirring motion within the melt inside mold 18 at an early stage of solidification. In the depicted embodiment only one stirrer 24 is arranged about mold 18 to induce rotational stirring of the melt in mold 18. Stirrer 24 could be replaced with a plurality (e.g. 2) of electromagnetic stirrers arranged about mold 18.

Additional at least two electromagnetic stirrers 26, 28 are positioned downstream of mold 18 about cast strand 22, at chosen positions detailed below. Again, stirrers 26, 28 are typically enclosed in a housing (not shown), and co-located in this housing.

At distances away from and downstream of mold 18, cast strand 22 progresses in its solidification, resulting in a shell of increasing thickness, while the central core of cast strand 22, remains substantially unsolidified as illustrated in FIGS. 1 and 4. The temperature of melt 41 within cast strand 22 declines progressively with time and distance away from mold 18, and, at a certain point, the temperature at the centerline of cast strand 22 crosses under the liquidus temperature for the particular molten material being cast. This point on the centerline of cast strand 22 is illustrated by numeral 48 in FIG. 1.

As the temperature in melt 41 declines below the liquidus temperature, the solid phase in the form of both free suspended crystals and crystalline cohesive network starts to form throughout the volume of melt 41. The mixture of the liquid and solid phases is commonly termed the “mushy zone” of melt 41, and is identified as zone 30. The region of cast strand 22, including a solidified shell and mushy zone of melt 41 is referred to as a transitional region of cast strand 22. Formation of a crystalline network in zone 30 typically results in shrinkage porosity, fissures, elemental macrosegregation, and the like, in cast product and may thus affect the quality of the cast product.

An example distribution of liquid and solid along the length of cast strand 22 is depicted in FIG. 4. The graph illustrates the solid fraction of melt 41, graphed against the thickness of the outer shell. Mushy zone 30 occupies the region between liquid and solid. As illustrated, the solid fraction increases radially away from the centre axis of cast strand 41, and along the length of cast strand 22, away from the meniscus of melt 41.

Conveniently, turbulence in the transitional region will disrupt the formation of a crystalline network, break-up dendrites into smaller fragments and homogenize melt 41 in mushy zone 30, at least in part, resulting in in more refined, less porous, and more homogeneous solidification structure and thus improvements of quality of the cast products. However, although conventional rotational stirring essentially produces turbulence at the solid-liquid interface, it has small impact on mixing throughout melt 41.

As such, in the depicted embodiment, additional first and second stirrers 26, 28 are positioned along cast strand 22 at a position corresponding to mushy zone 30. In particular, stirrers 26, 28 may be positioned to disrupt the crystals and crystalline structure in mushy zone 30. To this end, stirrers 26, 28 may be positioned at a location along the length of cast strand 22, where the temperature along the central axis of melt 41 is below the liquidus temperature and where 10 to 20 volumetric percent of melt 41 has substantially solidified, while the remaining 80 to 90 volumetric percent remains in a substantially liquid state in which the substantially solidified material is mixed. The volumetric percent of mushy zone 30 and its spatial distribution within a particular solidifying melt 41 along strand 22 may be determined by numerical computer simulation using solidification models. Such simulation may be combined in some instances with real-time measurements of major casting variables including casting speed, intensity of the primary and secondary cooling, and the like, which may provide data to improve modelling accuracy.

In the depicted embodiment, only two stirrers 26, 28 are illustrated downstream of mold 14. A person of ordinary skill will however appreciated that more than two stirrers could be located downstream of mold 14, in order to disrupt the crystals and crystalline structure in mushy zone 30.

FIG. 2 shows an enlarged schematic of cast strand 22 of FIG. 1, proximate first and second stirrer 26, 28. As illustrated, stirrers 26 and 28, may be arranged in proximity to each other at a predetermined distance L along the lengthwise extent of cast strand 22 about zone 30. L may for example, be in the decimetre to meter range. For instance L may be about 0.2 m.

Each of stirrers 24, 26, 28 may, for example, be formed as an inductor, including a stator 32 made of ferromagnetic or similar material, excited by a plurality of winding coils 36, wound about poles 34, as depicted in FIG. 3. One or more controlled A.C. electric power sources (not shown) may be interconnected with windings 36 to apply an electric current to each winding 36. Currents applied to windings 36 are poly-phase, with currents applied to opposite poles 34 being in phase with each other. The applied currents result in a rotating magnetic field in the volume encompassed by stator 32. Conveniently, the exact structures of stirrers 24, 26, and 28 may be identical or may differ, with each stirrer 24, 26, 28 having its own number of pole pairs, windings, size, and power source. For example, stirrers 26, 28 may each have three pole pairs; alternatively one could have two pole pairs, and the other three. Other combinations will be apparent to those of ordinary skill. Similarly, the longitudinal extent along cast strand 22, of each stirrer 26, 28 may differ from the longitudinal extent of the others.

In operation, stirrer 24 is energized to stir molten material in mold 18 (FIG. 1). Stirrers 26, 28 are also energized to each generate a rotating magnetic field having a common axis of magnetic field rotation. This axis of magnetic field rotation may be parallel, but need not necessarily coincide with, the central axis of cast strand 22. Specifically, each of windings 36 (FIG. 3) of stirrers 26, 28 is energized by an A.C. polyphase, single frequency electric current supplied from one or more independent power sources (not shown), also controlled by a controller. This electrical arrangement provides independent control of the magnetic fields (and thus independent rotating magnetic fields) produced by each respective stirrer 26, 28. As a consequence the magnetic flux density produced by first and second stirrers 26, 28 may be the same or different. The difference in magnetic flux densities may be constant or vary in time.

Rotational directions of magnetic fields of stirrers 26 and 28 may coincide, as denoted by the arrows B and C in FIG. 2, or oppose each other, as indicated by the arrows of A and C. Direction and angular velocity of rotation may be selected by an operator.

The alternating electric currents supplied to windings 36 of stirrers 26, 28 generate a rotational electromagnetic field, having a frequency within the range of about 1 to about 60 Hz, depending on stirring application. For many common applications, such as continuous casting of steel billets and blooms, frequencies within 5 to 30 Hz may be used. In the depicted embodiment, the frequency of the field of one stirrer 26 differs from the frequency of the other stirrer 28 by a certain predetermined value in order to produce a modulated magnetic field. The frequency difference may vary in time or be time independent and remain constant. The range of frequency variation may be between about 0.1 and 3.0 Hz (i.e. less that 3.0 Hz). A modulated magnetic fields resulting from superposition of the original magnetic fields produced by the respective adjacent stirrers is predominant, but not limited, in the region between the adjacent stirrers 26, 28 denoted by L in FIG. 2. The magnetic force produced by these superposed magnetic fields is the result of interaction between the magnetic fields of each stirrer 26, 28 and the currents induced by these magnetic fields in melt 41. The magnetic force will have multiple terms, and may create turbulence within melt 41 in mushy zone 30.

Specifically, the magnetic flux density and the current induced in melt 41 are mostly confined between the adjacent inductors will be the vector sums of the respective contributions of each inductor, as a result of superposition of their respective magnetic fields, as shown in FIG. 5. The magnetic force produced within melt 41 will be the vector product of the total magnetic flux density and total current density: {right arrow over (f)}={right arrow over (J)}×{right arrow over (B)}. Since the magnetic flux and current densities are composed of two contributions from two adjacent stirrers 26, 28 the magnetic force will have multiple terms.

Fundamentally this force will have two constant, or DC, terms and two double frequency terms. In addition, there are present two time varying terms involving the sum of original magnetic field angular frequencies (ω₁+ω₂) and two time varying terms involving the angular frequency difference, i.e. (ω₁−ω₂). The double frequency and the frequency sum components of magnetic force or torque typically have little impact on flow in the melt 41 due to the inertial effects of melt 41. The magnetic force or torque of the component having frequency (ω₁+ω₂) varies sufficiently slowly in time, to overcome inertia of melt 41. Since the induced current in melt 41 is proportional to the comparatively large angular frequency of the original magnetic fields, the magnitude of magnetic force and torque will also be large. At the same time, the low frequency time variation resulting from the frequency difference between the two magnetic fields will create large amplitude oscillations of the modulated force, which, in turn, will cause the angular velocity variations. The impact of modulation on stirring velocity increases with modulation frequency decrease.

As will be appreciated in the event more than two stirrers are positioned about mushy zone 30, the superposition of the multiple independent rotating fields of the multiple stirrers may create the desired turbulence.

Although the magnetic force will have high and low frequency components, only low frequency components will typically impact melt 41, due to the inertia of melt 41 (also referred to as inertial filtering by melt 41). FIGS. 6 and 7 illustrate magnetic force resulting from the superposition of two magnetic fields of the same rotating direction. As illustrated the amplitude of the modulated magnetic force per unit oscillates between 0 and 4, where 1 is the amplitude of non-modulated, steady state force associated with either one of the original magnetic fields. After high frequency components of the modulated force are filtered by the inertia of melt 41, the low frequency force variation oscillates, for example, in the range of +/−20 percent of the average force amplitude, as shown in an example in FIG. 7. The stirring produced by this force may also be characterized by large oscillations of primary and secondary flows. An example of angular velocity oscillations of the stirring is shown in FIG. 8.

FIG. 9 is a graph of depicting angular stirring velocity produced by different modes of stirring. The velocity profile denoted by A produced by stirring with two identical magnetic fields of the same rotating direction. The velocity profile denoted by B is produced by the same stirring conditions as in A, except the frequency of the respective magnetic fields differ by 0.5 Hz, i.e. f₁=18.0 Hz and f₂=17.5 Hz. The velocity profile denoted by C is produced by two magnetic fields with opposite rotating directions. The frequencies of respective counter-rotating magnetic fields are: f₁=18.0 Hz and f₂=17.5 Hz. The arrows under velocity profile C indicate a counter-rotating stirring motion in the stirring pool.

As illustrated in FIG. 9, in cases where counter-rotating fields are applied, the angular velocity of the counter-rotating stirring denoted by C, may be substantially reduced when compared to the velocity of unidirectional stirring flow produced by magnetic fields of the same frequency (marked by A) or different frequencies, as in the case denoted by B. Convenient, the reduced stirring velocity does not have a negative impact on stirring because the flow kinetic energy is transformed into turbulence. The counter-rotating stirring flows collide in the region between stirrers 26, 28 resulting in steep gradients of angular velocity caused by decline of velocity of one direction followed by a similar rapid recovery of the opposite direction velocity, as shown in FIG. 9. This oscillating nature of angular and axial-radial velocity components is indicative of turbulence intensity. FIG. 10 further shows an example of angular velocity oscillations measured in a column of mercury with induced counter-rotating stirring. The large changes in oscillation result from a combined action of the modulated magnetic field and stirring flows of the opposing directions which are produced by the counter-rotating magnetic fields of the adjacent stirrers 26, 28.

FIG. 11 depicts oscillating velocity in the central axial direction obtained by 3-dimensional numerical simulation in an example melt of steel. The velocity profiles shown correspond to the locales in melt 41 identified in FIG. 12. Large velocity oscillations are known to be indicative of highly turbulent flow induced by EMS in melt 41. Turbulence intensity may be qualitatively characterized by turbulent viscosity. FIGS. 13 and 14 further show example turbulent viscosities at different locations of the stirring pool. FIG. 13 shows turbulent viscosity at the stirring pool center, at the locales in FIG. 12. As illustrated in FIG. 13, the highest intensity turbulence occurs at the mid-distance between the adjacent inductors (the locale III in FIG. 12). In comparison, turbulence intensity at the same locale of the stirring pool produced by conventional unidirectional rotating stirring is shown in FIG. 14. As illustrated, the turbulence created by the counter-rotating stirring in the example melt is up to 5 times greater, having peaks characterized in turbulent viscosity in excess of 2 Ns/m², and often in excess of 2.5 Ns/m².

As an alternative to the application of electromagnetic fields of the same rotational direction, counter-rotating magnetic fields may be generated at stirrers 26, 28. Counter-rotating magnetic fields produced by adjacent stirrers 26, 28 will excite the counter-rotating flows within melt 41 in zone 30 which collide in the space between adjacent stirrers 26, 28. As a result of this flow collision, a steep gradient of declining angular velocity in one rotating direction will be followed by a similar gradient due to increasing velocity in the opposite rotating direction. In addition, the angular velocity also exhibits large oscillations. Both these primary flow characteristics, i.e. velocity gradients and oscillations, contribute to generating strong oscillatory recirculating flows in the axial-radial plane. Numerical simulations confirm the presence of the flows in the melt 41, particularly at the locations in FIG. 12. High intensive turbulence and shear stress, especially along the axial and radial directions of cast strand 22, will develop, particularly within the volume of melt 41 in the region between the adjacent stirrers 26, 28.

Additional turbulence in the region between stirrers 26, 28 may result from the electromagnetic forces originated from the superposition of counter-rotating magnetic fields of different frequency. As noted, low frequency oscillating magnetic forces from magnetic field modulation will generate perturbations in melt 41, which might become especially significant if those frequencies are within the range of the melt natural frequency due, for example, to the effect of parametric resonance of the melt. In addition, other modulation parameters, such as electric current amplitude and phase angle variations, can further enhance the modulated forces when compared to non-modulated, time averaged magnetic forces, and consequently, increase turbulence intensity and its effect on improvements in the solidification structure. Proximate stirrers 26, 28 provide for strong modulated magnetic forces resulting from superposed magnetic fields of either common or opposing rotating directions produced by the conventional design equipment, i.e. inductors and power sources.

Conveniently, an increase in turbulence in melt 41 will result in effective disruption of the crystalline network and mixing the crystals along with the solute enriched central region of the melt with the rest of the bulk. As a result, solidification structure and overall quality of the cast products will be improved.

As will now be readily appreciated, although EMS system 12 has been depicted as including two EMS stirrers 26 and 28 arranged to generated a modulated magnetic field, such a field could be generated with three or more stirrers, generating superposing rotating magnetic fields.

As may now be apparent, modulated electromagnetic stirring, exemplary of embodiments of the present invention, may be used in most casting and foundry process, where the cast product dimensions and geometry allow for producing rotating flow within a solidifying melt. In the case of a stationary casting, for example, a modulated electromagnetic stirring system may initially produce unidirectional magnetic fields and therefore unidirectional rotating swirl flow at an early solidification stage. At a certain predetermined time, the stirring system may be switched into counter-rotating stirring mode of operation, to generate turbulence at an advanced stage of solidification. Some rheocasting processes can similarly benefit from such modulated stirring.

Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims. 

1. A method of electromagnetic stirring a molten metallic material comprising: providing at least two stirrers for generating independent rotating magnetic fields about an axis extending through said molten material; wherein at least first and second ones of said at least two stirrers produce independent first and second rotating magnetic fields have differing angular frequencies; and wherein said stirrers are located about said molten metallic material in sufficiently close proximity to each other so that said independent rotating magnetic fields superpose to produce a modulated magnetic field that creates a turbulent flow of said molten metallic material in a transition region of said molten metallic material having a temperature below the liquidus along a central axis of said molten metallic material, and in which said molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material.
 2. The method of claim 1, wherein said first and second rotating magnetic fields counter-rotate.
 3. The method of claim 1, wherein said first and second rotating magnetic fields rotate in the same direction.
 4. The method of claim 1, wherein the longitudinal extent of said first of said at least two stirrers about said molten metallic material is different from the longitudinal extent of said second of said at least two stirrers about said molten metallic material.
 5. The method of claim 2, wherein the frequency of said first and second rotating magnetic fields differ by less than about 3 Hz.
 6. The method of claim 2, wherein the difference in the frequency of said first and second rotating magnetic fields varies in time.
 7. The method of claim 1, wherein each of said stirrers comprises at least two pole pairs, each excited by a current from at least one multi-phase current source.
 8. The method of claim 2, wherein said molten metallic material is within a cast strand, downstream of a casting mold.
 9. The method of claim 1, wherein each of said first and second ones of said at least two stirrers produce a different magnetic flux density in said molten metallic material.
 10. The method of claim 1, wherein the magnetic flux density produced in said molten metallic material produced by at least one of said first and second of said at least two stirrers varies in time.
 11. The method of claim 1, wherein said turbulent flow has a turbulent viscosity having peaks in excess of 2 Ns/m²
 12. The method of claim 1, wherein said region comprises substantially liquid molten metallic material, and crystalline material surrounded by a solid shell.
 13. The method of claim 1, wherein said turbulent flow disrupts formation of a crystalline network in said region.
 14. The method of claim 1, further comprising transferring molten metallic material through a mold upstream of said region, and providing a further stirrer about said mold to generate a rotating magnetic field within said mold.
 15. A casting apparatus comprising: a mold for casting a molten metal; a first stirrer for generating a first rotating magnetic field about an axis extending through said molten metal, located downstream of said mold; a second stirrer for generating a second rotating magnetic field, located downstream of said first stirrer; at least one power source for generating said first and second magnetic field, at frequencies of rotation differing from each other; wherein said first and second stirrers are arranged in proximity to each other so that said first and second rotating magnetic fields produce a modulated magnetic field that creates a turbulent flow in a molten metallic material in a region between said first and second stirrers.
 16. The apparatus of claim 15, wherein said first and second rotating magnetic fields are generated to counter-rotate by said at least one power source.
 17. The apparatus of claim 15, wherein said first and second rotating magnetic fields are generated to rotate in the same direction by said at least one power source.
 18. The apparatus of claim 15, wherein the longitudinal extent of said first stirrer about said molten metal is different from the longitudinal extent of said second stirrer about said molten metal.
 19. The apparatus of claim 16, wherein the frequencies of said first and second rotating magnetic fields differ by less than about 3 Hz.
 20. The apparatus of claim 16, wherein the difference in the frequency of said first and second rotating magnetic fields varies in time.
 21. The apparatus of claim 15, wherein each of said first and second stirrer comprises at least two pole pairs, each excited by a current from said at least one source.
 22. The apparatus of claim 15, wherein each of said first and second stirrers produce a different magnetic flux density in said molten metal.
 23. The apparatus of claim 15, wherein the magnetic flux density produced in said molten metal produced by at least one of said first and second stirrers varies in time.
 24. The apparatus of claim 15, wherein said turbulent flow has a turbulent viscosity having peaks in excess of 2 Ns/m²
 25. The apparatus of claim 15, wherein said region comprises substantially liquid molten metal, and crystalline material surrounded by a solid shell.
 26. The apparatus of claim 15, wherein said turbulent flow disrupts formation of a crystalline network in said region.
 27. The apparatus of claim 15, further comprising a further stirrer about said mold to generate a rotating magnetic field within said mold.
 28. A method of electromagnetic stirring a metallic melt comprising: providing a first stirrer for generating a first rotating magnetic field that rotates about an axis extending through said melt, at an angular frequency of ω₁; providing a second stirrer for generating a second rotating magnetic field that rotates at an angular frequency of ω₂; wherein said first and second stirrers are located in sufficiently close proximity to each other so that said first and second rotating magnetic field produce a magnetic force having a component with frequency (ω₁−ω₂) in said metallic melt in a region between said first and second stirrer, wherein (ω₁−ω₂) is sufficiently small to allow said magnetic force to overcome the inertia of said melt.
 29. A method of electromagnetic stirring a molten metallic material comprising: providing a first stirrer for generating a first rotating magnetic field about an axis extending through said molten material; providing a second stirrer for generating a second rotating magnetic field having a frequency of rotation differing from said first rotating magnetic field; wherein said first and second stirrers are located about said molten metallic material in sufficiently close proximity to each other so that said first and second rotating magnetic fields superpose between said first and second stirrers to produce a modulated magnetic field that creates a turbulent flow of said molten metallic material in a transition region of said molten metallic material having a temperature below the liquidus along a central of the molten metallic material, and in which said molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material. 